Continuous carbon fibers and graphite fibers are produced from pitch, polyacrylonitrile (PAN), and rayon. Most carbon fibers (about 90%) are made from PAN fibers and only a small amount (about 10%) is manufactured from petroleum pitch or rayon. Although the production of carbon fibers from different precursors requires different processing conditions, the essential features are similar. Generally, carbon fibers are manufactured by a controlled pyrolysis of stabilized precursor fibers. Precursor fibers (e.g. PAN) are first stabilized at about 200-400° C. in air by an oxidization process. The resulting infusible, stabilized fibers are then subjected to a high temperature treatment at approximately 1,000-1,500° C. (up to 2,000° C. in some cases) in an inert atmosphere to remove hydrogen, oxygen, nitrogen, and other non-carbon elements. This step is often called carbonization and it can take 2-24 hours to complete, depending upon the carbonization temperature and the starting material used. Carbonized fibers can be further graphitized at an even higher temperature, up to around 3,000° C. to achieve higher carbon content and higher degree of graphitization, mainly for the purpose of achieving higher Young's modulus or higher strength in the fiber direction, but not both. This takes another 1-4 hours under strictly controlled atmosphere and ultra-high temperature conditions. The properties of the resulting carbon/graphite fibers are affected by many factors, such as crystallinity, crystallite sizes, molecular orientation, carbon content, and the type and amount of defects.
Specifically, the carbon fibers can be heat-treated to become high modulus graphite fibers (from pitch) or high strength carbon fibers (from PAN-based). Carbon fibers heated in the range of 1500-2000° C. (carbonization) exhibits the highest tensile strength (5,650 MPa), while carbon fiber heated from 2500 to 3000° C. (graphitization) exhibits a higher modulus of elasticity (531 GPa). The tensile strength of carbon/graphite fibers is typically in the range of 1-6 GPa, and the Young's modulus is typically in the range of 100-588 GPa.
Broadly speaking, in terms of final mechanical properties, carbon/graphite fibers can be roughly classified into ultra-high modulus (>500 GPa), high modulus (>300 GPa), intermediate modulus (>200 GPa), low modulus (100 GPa), and high strength (>4 GPa) carbon fibers. Carbon fibers can also be classified, based on final heat treatment temperatures, into type I (2,000° C. heat treatment), type II (1,500° C. heat treatment), and type III (1,000° C. heat treatment). Type II PAN-based carbon fibers are usually high strength carbon fibers, while most of the high modulus carbon fibers belong to type I from pitch.
Regardless the type of carbon fibers or graphite fibers desired, the production of continuous carbon fibers and graphite fibers from pitch, PAN, and rayon is a tedious, energy-intensive, very challenging (requiring extreme temperature and atmosphere control), and expensive process. A strong need exists for a facile, less energy-intensive, simpler and more scalable, and more cost-effective process for producing advanced graphite fibers, fiber tows and yarns, and composites.
Carbon is known to have five unique crystalline structures, including diamond, fullerene (0-D nano graphitic material), carbon nano-tube or carbon nano-fiber (1-D nano graphitic material), graphene (2-D nano graphitic material), and graphite (3-D graphitic material, including graphite fiber). The carbon nano-tube (CNT) refers to a tubular structure grown with a single wall or multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of a few nanometers to a few hundred nanometers. Their longitudinal, hollow structures impart unique mechanical, electrical and chemical properties to the material. The CNT or CNF is a one-dimensional nano carbon or 1-D nano graphite material. Although multiple CNTs or CNFs can be spun into fiber yarns, these yarns are not considered as “continuous fibers”. They are twisted aggregates of individual CNTs or CNFs (each being but a few microns long) that are not self-bonded together; instead, they are mechanically fastened together as a yarn.
Bulk natural graphite is a 3-D graphitic material with each particle being composed of multiple grains (a grain being a graphite single crystal or crystallite) with grain boundaries (amorphous or defect zones) demarcating neighboring graphite single crystals. Each grain is composed of multiple graphene planes that are oriented parallel to one another. A graphene plane in a graphite crystallite is composed of carbon atoms occupying a two-dimensional, hexagonal lattice. In a given grain or single crystal, the graphene planes are stacked and bonded via van der Waal forces in the crystallographic c-direction (perpendicular to the graphene plane or basal plane). Although all the graphene planes in one grain are parallel to one another, typically the graphene planes in one grain and the graphene planes in an adjacent grain are different in orientation. In other words, the orientations of the various grains in a graphite particle typically differ from one grain to another.
A graphite single crystal (crystallite) per se is anisotropic with a property measured along a direction in the basal plane (crystallographic a- or b-axis direction) being dramatically different than if measured along the crystallographic c-axis direction (thickness direction). For instance, the thermal conductivity of a graphite single crystal can be up to approximately 1,920 W/mK (theoretical) or 1,800 W/mK (experimental) in the basal plane (crystallographic a- and b-axis directions), but that along the crystallographic c-axis direction is less than 10 W/mK (typically less than 5 W/mK). Further, the multiple grains or crystallites in a graphite particle are typically all oriented along different directions. Consequently, a natural graphite particle composed of multiple grains of different orientations exhibits an average property between these two extremes; i.e. between 5 W/mK and 1,800 W/mK.
It would be highly desirable in many applications to produce a continuous graphitic fiber (containing single or multiple grains) having a sufficiently large length and having all graphene planes being essentially parallel to one another along one desired direction (e.g. along the fiber axis). For instance, it is highly desirable to have one long graphite filament (e.g. a fully integrated or unitary filament of multiple graphene planes) having all the constituent graphene planes being substantially parallel to one another along the fiber axis direction without forming a helical structure or a porous structure. It would be further desirable if such a long or continuous graphite fiber has only one grain or few grains (thus, no or little grain boundaries) and has few defects therein to impede the flow of electrons and phonons. Conventional graphite fibers are known to have graphite crystal (grain) sizes less than 200 nm, mostly less than 100 nm. It would be most desirable to have a graphitic fiber having a grain size along the fiber axis direction being larger than 10 μm, preferably larger than 100 μm, more preferably on the magnitude of mm in dimension, further preferably cm in dimension, still further preferably meters in dimension. Thus far, it has not been possible to produce this type of large grain-size unitary graphene entity (fiber) from existing natural or synthetic graphite particles. This is part of what we have accomplished in the instant invention.
The constituent graphene planes of a graphite crystallite in a graphite particle can be exfoliated and extracted or isolated from a graphite crystallite to obtain individual graphene sheets of carbon atoms provided the inter-planar van der Waals forces can be overcome. An isolated, individual graphene sheet of carbon atoms is commonly referred to as single-layer graphene. A stack of multiple graphene planes bonded through van der Waals forces in the thickness direction with an inter-graphene plane spacing of 0.3354 nm is commonly referred to as a multi-layer graphene. A multi-layer graphene platelet has up to 300 layers of graphene planes (<100 nm in thickness), but more typically up to 30 graphene planes (<10 nm in thickness), even more typically up to 20 graphene planes (<7 nm in thickness), and most typically up to 10 graphene planes (commonly referred to as few-layer graphene in scientific community). Single-layer graphene and multi-layer graphene sheets are collectively called “nano graphene platelets” (NGPs). Graphene sheets/platelets or NGPs are a new class of carbon nano material (a 2-D nano carbon) that is distinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.
Our research group pioneered the development of graphene materials and related production processes as early as 2002: (1) B. Z. Jang and W. C. Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4, 2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al. “Process for Producing Nano-scaled Graphene Plates,” U.S. patent application Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A. Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets and Nanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25, 2006).
NGPs are typically obtained by intercalating natural graphite particles with a strong acid and/or oxidizing agent to obtain a graphite intercalation compound (GIC) or graphite oxide (GO), as illustrated in FIG. 1(a) (process flow chart) and FIG. 1(b) (schematic drawing). The presence of chemical species or functional groups in the interstitial spaces between graphene planes serves to increase the inter-graphene spacing (d002, as determined by X-ray diffraction), thereby significantly reducing the van der Waals forces that otherwise hold graphene planes together along the c-axis direction. The GIC or GO is most often produced by immersing natural graphite powder (20 in FIG. 1(a) and 100 in FIG. 1(b)) in a mixture of sulfuric acid, nitric acid (an oxidizing agent), and another oxidizing agent (e.g. potassium permanganate or sodium perchlorate). The resulting GIC (22 or 102) is actually some type of graphite oxide (GO) particles. This GIC is then repeatedly washed and rinsed in water to remove excess acids, resulting in a graphite oxide suspension or dispersion, which contains discrete and visually discernible graphite oxide particles dispersed in water. This rinsing step may be followed by several different processing routes:
For instance, Route 1 involves removing water from the suspension to obtain “expandable graphite,” which is essentially a mass of dried GIC or dried graphite oxide particles. Upon exposure of expandable graphite to a temperature in the range of typically 800-1,050° C. for approximately 30 seconds to 2 minutes, the GIC undergoes a rapid expansion by a factor of 30-300 to form “graphite worms” (24 or 104), which are each a collection of exfoliated, but largely un-separated graphite flakes that remain interconnected. A SEM image of graphite worms is presented in FIG. 2(a).
In Route 1A, these graphite worms (exfoliated graphite or “networks of interconnected/non-separated graphite flakes”) can be re-compressed to obtain flexible graphite sheets or foils (26 or 106) that typically have a thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm). Flexible graphite (FG) foils can be used as a heat spreader material, but exhibiting a maximum in-plane thermal conductivity of typically less than 500 W/mK (more typically <300 W/mK) and in-plane electrical conductivity no greater than 1,500 S/cm. These low conductivity values are a direct result of the many defects, wrinkled or folded graphite flakes, interruptions or gaps between graphite flakes, and non-parallel flakes (e.g. SEM image in FIG. 2(b)). Many flakes are inclined with respect to one another at a very large angle (e.g. mis-orientation of 20-40 degrees).
Alternatively, one may choose to use a low-intensity air mill or shearing machine to simply break up the graphite worms for the purpose of producing the so-called “expanded graphite flakes” (108) which contain mostly graphite flakes or platelets thicker than 100 nm (hence, not a nano material by definition).
In Route 1B, the exfoliated graphite is subjected to high-intensity mechanical shearing (e.g. using an ultrasonicator, high-shear mixer, high-intensity air jet mill, or high-energy ball mill) to form separated single-layer and multi-layer graphene sheets (collectively called NGPs, 33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814. Single-layer graphene can be as thin as 0.34 nm, while multi-layer graphene can have a thickness up to 100 nm, but more typically less than 20 nm.
Exfoliated graphite worms, expanded graphite flakes, and the recompressed mass of graphite worms (commonly referred to as flexible graphite sheet or flexible graphite foil) are all 3-D graphitic materials that are fundamentally different and patently distinct from either the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbon material (graphene sheets or platelets, NGPs).
Route 2 entails ultrasonicating the graphite oxide suspension for the purpose of separating/isolating individual graphene oxide sheets from graphite oxide particles. This is based on the notion that the inter-graphene plane separation has been increased from 0.3354 nm in natural graphite to 0.6-1.1 nm in highly oxidized graphite oxide, significantly weakening the van der Waals forces that hold neighboring planes together. Ultrasonic power can be sufficient to further separate graphene plane sheets to form separated, isolated, or discrete graphene oxide (GO) sheets. These graphene oxide sheets can then be chemically or thermally reduced to obtain “reduced graphene oxides” (RGO) typically having an oxygen content of 0.001%-10% by weight, more typically 0.01%-5% by weight and, most typically and desirably, less than 2% by weight.
For the purpose of defining the claims of the instant application, NGPs include discrete sheets/platelets of single-layer and multi-layer graphene, graphene oxide, or reduced graphene oxide with an oxygen content of 0-10% by weight, more typically 0-5% by weight, and preferably 0-2% by weight. Pristine graphene has essentially 0% oxygen. Graphene oxide (including RGO) can have approximately 0.001%-50% by weight of oxygen.
The GO molecules in graphene oxide gel, to be described in detail later, typically contain 20-50% by weight oxygen (more typically 30-47%) immediately after removal of the liquid from the GO gel, but prior to a subsequent heat treatment. The GO gel refers to a homogeneous solution of highly hydrophilic aromatic molecules (graphene oxide molecules bearing oxygen-containing groups, such as —OH, —COOH, and >O, on molecular planes or at the edges) that are dissolved (not just dispersed) in a liquid (e.g. acidic water). The GO gel per se does not contain visibly discernible or discrete graphene or GO particles in the form of solid sheets or platelets dispersed in the liquid medium. These GO molecules and the dissolving liquid medium have comparable indices of refraction, making the resulting gel optically transparent or translucent (if the proportion of GO molecules are not excessively high; e.g. <2% GO), or showing lightly brown color. In contrast, the simple mixture of original graphite particles or discrete graphene sheets/platelets with acids and/or water appears optically dark and totally opaque (even with only <0.1% solid particles suspended in the liquid medium). These particles or NGP platelets are simply dispersed (not dissolved) in the fluid medium.
These GO molecules in a GO gel are highly reactive and may be considered as “living giant molecules” or “living chains” By contrast, the prior art solid sheets/platelets of graphene, GO, and RGO are essentially “dead” species. The GO gel can be formed into a shape with a proper shearing or compression stress (e.g. via casting or extrusion through a tapered-diameter nozzle), dried (with liquid components partially or totally removed), and heat-treated under certain conditions to obtain a unitary graphene material (e.g. a continuous filament of the instant invention), which is typically a single crystal, a poly-crystal with incomplete or poorly delineated grain boundaries, or a poly-crystal with very large grain sizes (very few grains). The heat treatment serves to chemically link these active or living GO molecules to form a 2-D or 3-D network of chemically bonded graphene molecules of essentially infinite molecular weights, and to drastically reduce the oxygen content of GO down to below 10% by weight, more typically <5%, further more typically <2%, and most typically <<1%. Only a trace amount of oxygen (practically 0%) can survive if the heat treatment temperature is sufficiently high (>2,500° C.) and heat treatment time sufficiently long. This new and unique material called “unitary graphene material” in a continuous filament form will be further described in detail later. When in a filamentary form as disclosed herein, this unitary graphene material is a nearly perfect graphitic fiber.
Solid or “dead” NGPs (including discrete sheets/platelets of pristine graphene, GO, and GRO), when packed into a film, membrane, or paper sheet (34 or 114) of non-woven aggregates, typically do not exhibit a high thermal conductivity unless these sheets/platelets are closely packed and the film/membrane/paper is ultra-thin (e.g. <1 μm, which is mechanically weak). This is reported in our earlier U.S. patent application Ser. No. 11/784,606 (Apr. 9, 2007). In general, a paper-like structure or mat made from platelets/sheets of graphene, GO, or RGO (e.g. those paper sheets prepared by vacuum-assisted filtration process) exhibit many defects, wrinkled or folded graphene sheets, interruptions or gaps between platelets, and non-parallel platelets (e.g. SEM image in FIG. 3(b)), leading to relatively poor thermal conductivity, low electric conductivity, and low structural strength.
In a recent report [Z. Xu & C. Gao, “Graphene chiral liquid crystals and macroscopic assembled fibers,” Nature Communications, 2, 571 (2011)], graphene oxide sheets can form chiral liquid crystals in a twist-grain-boundary phase-like model with simultaneous lamellar ordering and long-range helical frustrations. Aqueous graphene oxide liquid crystals can then be continuously spun into meters of macroscopic graphene oxide fibers, which are chemically reduced to obtain RGO fibers. During the spinning process for GO fibers, the GO dispersions were loaded into glass syringes and injected into the NaOH/methanol solution under the conditions of 1.5 MPa N2. The NaOH/methanol solution is a coagulation solution (a non-solvent for GO) and the GO sheets are precipitated out as discrete/isolated sheets that are mechanical fastened in the fiber form as soon as the GO dispersions came in contact with the non-solvent in a coagulation bath. The fibers produced in the coagulation bath were then rolled onto a drum, washed by methanol to remove the salt, and dried for 24 hours at room temperature. The as-prepared GO fibers were then chemically reduced in the aqueous solution of hydro-iodic acid (40%) at 80° C. for 8 hours, followed by washing with methanol and vacuum drying for 12 hours.
Clearly, this is a very tedious and time-consuming process. Further, the GO sheets must be dispersed in water to a critical extent that they form chiral liquid crystals with a twist-grain-boundary phase structure in the GO suspension. This chiral or twist-grain boundary structure is a fatal defect as far as the mechanical strength of macroscopic graphene fibers is concerned, as evidenced by the relatively low tensile strength (102 MPa) reported by Xu, et al. This is three orders of magnitude lower than the intrinsic strength (130 GPa) of individual graphene sheets. Another severe problem of this process is the notion that the spinning-coagulation procedure inherently results in highly porous and non-oriented graphene sheets in the graphene fiber (e.g. FIGS. 2(c) and 2(d)). This porous and non-parallel graphene structure is another reason responsible for such a low tensile strength and low Young's modulus (5.4 GPa), which is almost three orders of magnitude lower than the theoretical Young's modulus of graphene (1,000 GPa).
A similar spinning-coagulation process was reported by Cong, et al [H. P. Cong, et al. “Wet-spinning assembly of continuous, neat, and macroscopic graphene fibers,” Scientific Report, 2 (2012) 613; DOI: 10.1038/srep00613]. Again, the reported tensile strength and Young's modulus of the graphene fibers are very poor: 145 MPa and 4.2 GPa, respectively. Slightly better tensile strength (180 MPa) was observed with graphene oxide fibers prepared by a confined-dimension hydrothermal method was reported [Z. Dong, et al. “Facile fabrication of light, flexible and multifunctional graphene fibers,” Adv. Mater. 24, 1856-1861 (2012)]. Even after a thermal reduction treatment, the maximum achievable tensile strength was only 420 MPa. Again, the graphene sheets in these graphene fibers, just like in the graphene fibers prepared by spinning-coagulation, remain discrete and poorly oriented. The fibers are also highly porous and of limited length. Furthermore, this process is not a scalable process and cannot be used to mass-produce continuous graphene fibers.
The helical structure and high porosity level of these conventional graphene fibers are a natural consequence of the liquid crystal structure of the starting graphene oxide material and the required precipitation of graphene from a liquid coagulation bath. Additionally, the graphene fibers obtained by drawing CVD graphene films into a fibrous form are also highly porous. These pores and helices severely weaken these conventional fibers, leading to dramatically lower elastic modulus and strength that what graphene could achieve. When used as a reinforcement phase, these weakened fibers also result in composites of poor mechanical properties.
In addition, there are several shortcomings associated with using conventional carbon or graphite fibers (abbreviated as CF or GF) as a reinforcing phase dispersed in a matrix material, such as a resin, in a composite. These shortcomings include:                (a) These fibers typically have either a nearly circular or irregular cross-section, not amenable to compact packing when they are combined to form a fiber tow. (A fiber tow is an untwisted aggregate of multiple continuous fibers; e.g. the 6 K, 12 K, and 24 K tow means a 6,000, 12,000, and 24,000 counts of continuous fibers, respectively, in a carbon/graphite fiber tow.) The limited packing factor of the fibers in a tow leads to a low fiber volume fraction in a matrix (maximum fiber volume fraction being 55%-65% in a resin matrix composite) and, hence, relatively low elastic modulus and low strength of the resulting composite.        (b) These conventional continuous carbon/graphite fibers, having a typical diameter of 6-12 μm, lead to a typically thick tow and ultimately a thick fabric layer. The resulting composite layer is typically hundreds of microns or even millimeter thick. In many applications, an ultra-thin composite layer (e.g. <50 μm or even <5 μm in thickness) is highly desirable.        (c) In a woven fabric composite, fabric quality and functional performance depends on the ability to inter-weave tows or yarns with one another before or after impregnation with a matrix material (e.g. resin). The material structure, size, and shape of the fibers and resulting tows may become limiting factors for the range of application of a certain fabric composite.        (d) A typical CF or GF fabric is made of CF or GF yarns and, in each yarn, constituent fibers cohere to form a nearly round cross section. Therefore, in a woven state, the cross section of the CF or GF yarn at the point at which the warp and weft cross each other is elliptic, with the weaving yarn being significantly crimped. This trend is conspicuous especially in a CF or GF fabric which uses carbon fiber yarns with a large yarn size. Hence, in the fabric with considerably crimped yarns, the fiber density tends to be non-uniform, preventing high strength of the CF or GF from being fully exploited. In addition, the fabric using CF or GF yarns with a large yarn or tow size is normally accompanied by a high woven fabric weight (g/m2) and increased thickness. This adversely affects the resin infiltration property when manufacturing a pre-impregnated material (hereinafter referred to simply as “prepreg”), or molding a fiber reinforced resin composite. Therefore, carbon fiber reinforced plastics (CFRP) produced by using a carbon fiber fabric woven with yarns of a large size inevitably leads to more voids present in the resin, preventing the realization of a high-strength composite.        (e) Conventional continuous carbon/graphite fibers typically have a hard carbon skin layer that is difficult to functionalize and, hence, it is difficult to achieve a strong interfacial bonding between the carbon/graphite fiber and the matrix resin. It normally requires the use of an undesirable surface treatment procedure (e.g. acid etching and plasma exposure) to improve the interfacial bonding and, in most situations, the surface treatment does not lead to a satisfactory result. This has been a long-standing problem associated with carbon/graphite fiber-resin composites.        (f) Generally, there are no available continuous fibers having a sub-micron or nanometer diameter/thickness and shape that provide significant strength, ductility, geometric flexibility, and cross-sectional shape of a yarn or tow so as to define a multi-functional fabric reinforced with a matrix material.        
Clearly, there is an urgent need for a new type of graphitic fibers that overcome most, if not all, of the aforementioned problems associated with conventional graphene-derived fibers and conventional pitch- and PAN-based carbon or graphite fibers for composite applications.
Our recent patent applications have provided a process for producing high-strength and high-modulus continuous graphitic fibers by using particles of natural graphite or artificial graphite as the starting material. Please refer to: A. Zhamu and B. Z. Jang, “Continuous Graphitic Fibers from Living Graphene Molecules,” U.S. patent application Ser. No. 13/986,223 (Apr. 15, 2013) and “Process for Producing Continuous Graphitic Fibers from Living Graphene Molecules,” U.S. patent application Ser. No. 13/986,208 (Apr. 15, 2013). Specifically, these patent applications have provided a graphene oxide gel-derived continuous graphitic fiber that is a unitary graphene material or monolithic graphene entity, not just an aggregate of discrete graphene or graphene oxide sheets. The GO gel-derived unitary graphene filaments exhibit a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, and elastic modulus unmatched by any continuous graphene fibers or carbon fibers. Specifically, these highly conductive, continuous graphitic fibers exhibit the following properties: (a) a thermal conductivity greater than 600 W/mK (typically greater than 1,000 W/mK, and can be greater than 1,700 W/mK); (b) an electrical conductivity greater than 2,000 S/cm (typically >3,000 S/cm, more typically >5,000 S/cm, often >10,000 S/cm, and even >15,000 S/cm); (c) a tensile strength greater than 1.2 GPa (typically >3.2 GPa, more typically >5.0 GPa, and can be >8.0 GPa); and/or (d) a Young's modulus greater than 60 GPa (typically >200 GPa, more typically >300 GPa, and often >600 GPa). No prior art continuous graphitic fiber meets this set of stringent technical requirements.
These exceptional properties of our continuous graphitic fibers are produced from living graphene chains by a unique and novel process without following the coagulation-spinning procedure or spinning from CVD graphene films. These new graphene fibers are generally flat-shaped in cross-section (non-circular, non-ellipsoidal, and non-oval shape), with a large width (typically from 0.01 μm to 20 μm and more typically from 0.1 μm to 10 μm, but readily adjustable) and a small thickness (typically from 1 nm to 1 μm, readily adjustable), hence a high width-to-thickness ratio (typically from 10 to 1000). They are relatively solid, not porous. These shapes, structures, and morphologies are in contrast to those graphene fibers produced by coagulation and spinning, which are helical and highly porous in nature and have a chiral or twist-grain boundary structure. These pores and helices severely weaken these conventional fibers, exhibiting dramatically lower elastic modulus and strength.
We have further observed that, due to the more or less rectangular cross-section of the presently invented continuous graphitic fibers, the tows or resin-impregnated tows containing multiple continuous fibers can have a cross-section that is rectangular or flat-shaped. When one combines multiple filaments together (e.g. of those conventional fibers with a circular cross-section or irregular-shape cross-section), there is a limit to the packing factor. The highest packing factor is typically between 50% and 65% by volume even for circular-cross-section fibers. In contrast, the presently invented rectangular or flat-shaped graphene fibers can be packed into a yarn with an essentially 100% packing factor. The packing factor can be adjusted to be between 20% and essentially 100%, for composite structure applications. A packing factor of 70-85% (that cannot be achieved with any conventional fibers) is particularly useful for composite applications. Our research data have demonstrated that the flexural strength and elastic modulus values of polymer matrix composites containing presently invented graphitic fiber-based tows as a reinforcement phase are significantly higher than those of the composites containing a comparable volume fraction of conventional graphitic fibers. The instant invention provides tightly packed tows. These features are not achievable with conventional graphitic fibers.